Why is therapeutic cloning used

These studies could lead to clinical applications for cancer diagnosis in humans since nuclear reprogramming signals from the host ooplasm variably reset the epigenetic profile of the nuclear donor DNA. The derivation through SCNT of a healthy patient-specific stem line would show that cancer onset was triggered by epigenetic alterations.

However, epigenetic resetting 15 following SCNT is likely to disrupt normal phenotype of the embryo-derived cell lines and the adult clone, the latter displaying an abnormally low body weight and expression level of MUP encoding genes Major Urinary Proteins as shown by Reik et al in the mouse. The epigenetic pattern of imprinted genes that was established during gametogenesis is lost through SCNT 20 and the inactivation of early genes directing embryogenesis can explain low embryo viability and poor efficiency in the derivation of autologous ntESC lines.

Blelloch et al found out, from studies on neurons, that stem cells used as the nuclear donor have a higher success rate 21 than fully differentiated cells in the derivation of autologous embryonic cells.

For instance, patient-specific cardiomyocytes produced through SCNT will not integrate into the scarred heart tissue resulting from myocardial infarction. For instance, Duchenne Muscular Dystrophy DMD is an inheritable X-linked condition characterized by reduced intramuscular dystophin levels, causing cellular necrosis and weakening Being a single-gene disorder, DMD can be treated by therapeutic cloning in combination with gene therapy to restore normal dystrophin production.

In the case where ntESC are transplanted without prior differentiation in vitro, the insertion of a transgene encoding MyoD 35 , a transcription factor responsible for commitment to the myogenic lineage, may promote muscle regeneration. The combination of gene therapy and therapeutic cloning has exciting potential for the genetic rescue of missing alleles in heritable genetic disorders such as severe combined immunodeficiency SCID , in which genetic mutations of specific genes such as RAG-1 and 2, essential for the DNA recombination allowing immunoglobulin and lymphocyte polymorphism, render the immune system completely inefficient.

Hochedlinger et al took a somatic nucleus from the tail-tip of an SCID mouse-model, created through the double-knockout of the Rag-2 gene recombination-activating gene 2 , and rescued the genetic defect through the insertion of two copies of the Rag-2 gene by homologous recombination However, mature T lymphocytes were not observed, suspected to be due to selective differentiation of the transplanted stem cells into myeloid cells bone marrow precursors instead Although more work needs to be done to elucidate the pathways leading to preferential differentiation in vivo, the combination of gene therapy for the rescue of a loss of function and therapeutic cloning to bypass graft rejection holds the potential to eventually cure other immune disorders.

Oncogenic activation following transduction constitutes a major drawback to this approach. In , the insertion of the transgene to treat X-linked SCID in the LMO2 oncogene caused the onset of leukemia in two out of seven patients recently treated Repeated graft rejection, even when derived through SCNT, remains an unsolved problem in the case of autoimmune disorders such as pernicious anemia and multiple sclerosis.

Legislative constraints and the subsequent lack of funding constitute a major impediment to the advancement of therapeutic cloning. For instance, although therapeutic cloning is not completely banned in the United States, federal funding is not permitted to be used in experiments involving the 20 cell lines in the NIH National Institute of Health registry 44 derived before August 9, Out of these cell lines approved by Bush, 12 died and the remaining is not useful for research purposes.

Researchers have to therefore rely on the scarcity of private funding, although 4 American states, including California with a yearly investment of millions, have a budget allowed specifically for stem cell research A major roadblock in the feasibility of human therapeutic cloning is the low availability of oocytes for research purposes.

Currently, due to low SCNT efficiency, it is estimated that human oocytes 35 would be needed in order to derive one observe patient-specific ntESC line. The Human Fertility and Embryo Authority, in England, allows women in fertility clinics to offer two oocytes for scientific research, provided that at least twelve oocytes are collected 25 , although the extra oocytes taken are more likely to be donated for in vitro insemination.

Substantial financial gain would incite poorer women to surrender of part of a finite supply of gametes, in addition to the risks incurred through surgerical removal of the oocytes and hormonal treatments. Ovarian hyperstimulation syndrome OHSS results, in most cases, from the administration of drugs such as gonadotropin-releasing hormone agonists 27 , to induce the simultaneous maturation of multiple follicles into oocytes. Fertility clinics are the major source of human oocytes for research.

The aged oocytes that did not fertilize during in vitro trials are not optimal for SCNT, as investigated by Hall et al, who observed overexpression of genes encoding for meiotic spindles proteins and a lower cleavage efficiency 30 of aged oocytes versus fresh ones. After two years of debate, Harvard is the only group currently allowed to use oocytes collected for the sole purpose of research, with informed consent.

Eggan and Melton intend to generate human ntESC lines from patients with diabetes, sickle cell anemia and amyotrophic lateral sclerosis ALS , a progressive neurodegenerative condition targeting motor neurons Interestingly, SCNT could provide a solution to low human oocyte availability and a promising therapeutic approach to circumvent infertility.

As reported by Nagy and Chang, artificial gametes 32 can be created by haploidization, through SCNT into an enucleated oocyte ready to undergo meiosis upon induction. Tesarik et al incorporated the nucleus of a human cumulus cell into an enucleated allogenic oocyte. However, abnormal chromosomal segregation and mitotic spindle assembly, as observed in mice, need to be resolved before haploidization through SCNT can be safely wide-spread in fertility clinics.

Once the molecular pathways of oocyte maturation are resolved, immature follicles could be collected postmortem 39 and induced to mature in vitro for research purpose. However, this option needs to be rigorously regulated, and is likely to stir an ethical controversy. An alternative to low human oocyte availability would be to use an oocyte of a different species. Successful trials were done to generate blastocysts in vitro through the SCNT of skin fibroblast nucleus from different mammals ungulates, rodents, pigs, monkey 34 and human into an enucleated bovine oocyte.

The insertion of human nuclear genome into an animal oocyte, especially through electrofusion where both human and animal mitochondrial DNA coexist in the same ooplasm, raise objections among the detractors of therapeutic cloning, although the percentage of total residual animal DNA nuclear and mitochondrial is too low to consider the hybrid as a chimera.

Immune rejection of the ntESC in cell replacement therapy is due to mitochondrial heteroplasmy as a consequence of SCNT since the nuclear donor and ooplasmic host cells are not autologous in most cases.

Mitochondrial heteroplasmy is also a major cause of SCNT embryo inviability beyond the eight-cell stage because the mitochondrial-nucleus interactions necessary for the production of most mitochondrial proteins are disrupted due to inter-species incompatibility Also, antigens such as Mta are encoded by the mitochondrial genome and trigger an autoimmune response targeting the hybrid 36 after transplantation.

Inter-species incompatibility can be circumvented to a certain degree if the donor and host species are closely related. For instance, embryonic cells derived from the injection of a human nucleus in a chimpanzee ooplasm were viable, contrarily to when SCNT was done with the ooplasm of a nonhuman primate such as the orangutan The transfer of mitochondria isolated from patient-specific biopsies 39 might circumvent the immune rejection problem due to mitochondrial heteroplasmy, and a female patient could in theory donate both the somatic nucleus and oocyte necessary for cell replacement therapy in her own body.

The latter option offers great promises since recent studies in bovine showed that autologous SCNT embryonic production was more efficient than when the nuclear donor and ooplasmic host are from allogenic origins, and epigenetic reprogramming occurs to a significantly less extend in autologous SCNT embryos The interspecies transmission of pathogens is a nonnegligible issue when injecting a human nucleus into the oocyte of another species, such as bovine or pig For instance, the porcine endogenous retrovirus PERV , although inoffensive in pigs, disrupts the transcription initiation of genes in humans, as demonstrated in vitro by Moalic et al, by integrating within the CpG islands of promoters Gene therapy approaches are being designed to reduce the infectivity of PERV due to the mannose-rich N-glycan integrated in the viral capsule Thus, through the insertion of genes such as ManIb and ManII, encoding mannosidases involved in N-glycan catabolism, the infectivity of PERV was significantly reduced but not annihilated in human cells.

Animal serum and non-proliferative mouse fibroblast used as feeder-cell layer to direct the development of the human ntESC 44 is problematic since animal contaminants can be transferred to the ntESC which in turn might trigger an immune response post-transplantation, thereby revoking the goal of therapeutic cloning. N-glycolylneuraminic acid Neu5Gc is a mammalian sialic acid a sugar with acidic side-chains present in the membrane of all cells not found in humans, although most of us have anti-Neu5Gc antibodies.

When human ntESC are grown on animal feeder cells or in contact with animal-derived serum, the stem cells incorporate enough Neu5Gc to potentially elicit an immune response 45 in vivo, hence killing the transplanted cells. Although the murine leukemia virus is not pathogenic 44 when transmitted from mouse feeder cells to human ntESC, non-cellular matrices are being designed to counteract the problem of animal contaminants.

Neu5Gc was not reported in the cell lines cultured on this human matrix. In sum, the issue of pathogenic transmission is in the process of being solved, bringing one step further the potential for clinical application of therapeutic cloning in cell replacement therapy. NtESC are subjected to the same tumorigenicity potential as wild-type stem cells. The formation of teratomas, after in vivo transplantation, is due to co-purification of pluripotent stem cells along with the wanted differentiated cells.

The teratomas resulted from a low concentration of 0. Although the hyperglycemia associated with type I diabetes was reversed, tumorgenesis occurred 20 days post-transplantation, rendering stem cells, whether wild-type or issued from therapeutic cloning, a non-viable option for clinical applications in this instance, unless better isolation methods for the exclusive purification of differentiated stem cells are designed.

The apparition of the primitive streak directing polarized development confers to the two-week embryo a higher moral status 51 as a potential human organism, compared to the earlier embryo at the stage of a randomly-organized group of cells. Consequently, laws prohibiting the culture of embryos for more than two-weeks, which marks the onset of gastrulation and the formation of the primitive streak, are in vigor in several countries such as the United States, based on a decision of the British Warnock Commission 29 in The ethical debate on the moral impermissibility of deliberate destruction of an embryo can be circumvented by a new technique deviced by Chung et al.

They successfully derived human ESC from a single cell without destroying the blastocyst in the process 52 , using the same manipulations normally devoted to genetic screening in preimplantation embryos. This method seems to be promising for solving the ethical concern of killing a human embryo, rendering feasible the prenatal generation of individual-specific cell lines for use in regenerative medicine later on in life.

Skip to main content. The Value of Therapeutic Cloning for Patients. Share Print. There is some confusion surrounding use of the word "cloning. However, it is important to distinguish between that and other appropriate and important uses of the technology such as cloning specific human cells, genes and other tissues that do not and cannot lead to a human being therapeutic cloning.

These techniques are integral to the production of breakthrough medicines, diagnostics and vaccines to treat many diseases. They could also produce replacement skin, cartilage and bone tissue for burn and accident victims, and result in ways to regenerate retinal and spinal cord tissue. The nation's top scientists from The National Academies of Science and National Institutes of Health, as well as numerous Nobel Laureates attest to the scientific value of this research.

A February, report from the National Academies of Science concluded that while reproductive cloning is unsafe and should be banned, therapeutic cloning has sufficient scientific potential that it should be allowed to continue. Stem cell research will help scientists learn how to develop cells and tissue to cure disease. Over many years, scientists have demonstrated that they may learn how to induce these cells to differentiate into many different cell types.

Accomplishing that would enable scientists to create new, healthy cells and tissue for transplantation to replace damaged or dead tissue. The report concluded that MRT in humans is ethically permissible as long as certain conditions and principles are met. One condition is that treatment should be limited to women who are at risk of transmitting severe mitochondrial disease and because female embryos would result in heritable genetic modification, MRT research should be restricted initially to male embryos.

The field of genetics is moving rapidly with new techniques that focus on DNA manipulation in vivo resulting in the alteration of genes to correct mutations, introduce new genetic information, and remove specific DNA sequences. To address the societal issues surrounding genome editing, the International Summit on Human Gene Editing, hosted by the scientific academies of China, the United Kingdom and the U.

The summit concluded that somatic therapies based on genome editing should proceed under the existing FDA regulatory framework, but editing the human germline would be irresponsible to pursue at this time. Approved Cellular and Gene Therapy Products. Sometimes, there are no right or wrong answers, or even answers at all. Some variables which would be considered when discussing stem cells include:.

Therapeutic cloning Transplanting stem cells Adult stem cell transplants use a patient's own stem cells. Embryonic stem cells will always come from a donor — unless stem cells were collected from the patient as an embryo.